U.S. Pat. No.5,355,283 discloses a prior art thermally enhanced IC chip package, which includes a substrate with a central hole. The substrate is bonded to a heat sink, which can be made of copper or other materials having an acceptably high thermal conductivity, by an adhesive such as an epoxy adhesive. An IC chip is also bonded to the heat sink by the adhesive within the central hole, which is subsequently filled with an encapsulant.
The presence of the heat sink in such a conventional IC chip package as described above can provide good heat transfer away from the chip, particularly since most of the heat sink surface is exposed to the exterior of the package. However, it has some drawbacks. The differences in coefficient of thermal expansion between the heat sink and the substrate create stress in the adhesive layer. This is exaggerated by differences in heating and cooling rates. The heat sink is heated directly by conduction through a relatively large area of an adhesive patch. The substrate is heated less directly by conduction through the heat sink and through the encapsulant. Thus, there are likely to be cyclical temperature differences that exaggerate differences in expansion due to differences in coefficient of thermal expansion. Moreover, the processes to implement an integral heat sink into the substrate are relatively complex and expansive.
Besides, there will have some problems on thermal capability when applying the above IC chip package to a stacked IC chip package, such as an embodiment disclosed in U.S. Pat. No.6,380,615, the upper chip stacked on the other chip might not reach an efficient heat dissipation.